server: Don't attach thread input to the desktop.

Now that the desktop can be owned by a thread we need to explicitly
check for it in the thread attach check.
diff --git a/server/window.c b/server/window.c
index 8254793..5ceab49 100644
--- a/server/window.c
+++ b/server/window.c
@@ -426,7 +426,7 @@
     }
 
     /* if parent belongs to a different thread, attach the two threads */
-    if (parent && parent->thread && parent->thread != current)
+    if (parent && parent->thread && parent->thread != current && !is_desktop_window(parent))
     {
         if (!attach_thread_input( current, parent->thread )) goto failed;
     }